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Aerobic Degradation Potential Assessment from Oxygen and Carbon Dioxide Soil Gas Concentrations in Roadside Soil

Civil and Environmental Engineering Dep., Univ. of Massachusetts, Amherst, MA 01003;
Research and Materials Section, Massachusetts Highway Dep., South Boston, MA 02210.

^* Corresponding author (ostendorf{at}ecs.umass.edu).

ABSTRACT

The application of calcium magnesium acetate [CMA; Ca_0.3Mg_0.7-(C₂H₃O₂)₂] to highways as a deicing agent induces aerobic acetate degradation by microorganisms in the capillary fringe, which may reduce O₂ demand in the underlying groundwater. We assessed this aerobic degradation potential by computing the diffusive fluxes of O₂ and CO₂ from measured soil gas concentrations in the unsaturated zone of a mildly acidic roadside soil. The fluxes were relevant to the potential assessment because O₂ is consumed and CO₂ is generated by the aerobic degradation of organic substrates such as acetate. Soil gas O₂ and CO₂ were measured on a monthly basis in tubing clusters in the unsaturated zone over a 2-yr period using portable meters. Quasi-steady gaseous diffusion models were calibrated with the data, showing that O₂ diffused into the fringe at a rate that was proportional to the exiting diffusive flux of CO₂. The modeled fluxes were consistent with an independent estimate of acetate-based microcosm reaction rates in the capillary fringe. Roadside soils have the potential to significantly degrade CMA under aerobic conditions before the deicing agent reaches the water table, thus reducing O₂ demand in the groundwater.